The present invention relates to active antenna systems incorporated into vehicle windows, and more particularly to such systems including two or more antennas for different frequency ranges (such as AM and FM).
Active antenna systems are widely used in automotive applications. Such systems include mast antennas and window antennas, which may be integrated into the front, side, or rear windows of a vehicle. Increasingly, the rear window has become a candidate for antenna placement in view of recent aerodynamic design trends. Specifically, the window surface area has increased, particularly on the rear window where the glass is often mounted at a very low angle. Heating the entire rear window for visibility is not necessary, and accordingly, heating grids are typically located over only a portion, for example half, of the glass area. The non-heating grid portion of the window provides adequate space for antennas.
It is known to use the heating grid on one of the vehicle windows (usually the rear window) as an antenna for either AM or FM. In such cases, it is necessary to include a second antenna for the other of AM or FM. Fortunately, with the increased surface area on rear windows, there is adequate room for both a heating grid and a second antenna.
In a first approach, the heating grid is used as the FM antenna; and a flat AM antenna is incorporated on or in the glass above the heating grid. The system is optimized by both the configuration of the AM antenna and its spacing from the heating grid and the surrounding sheet metal. An example of such a system is illustrated in U.S. Pat. No. 4,791,426 issued Dec. 13, 1988 to Lindenmeier et al and entitled "Active Antenna in the Rear Window of a Motor Vehicle."
In a second approach, the heating grid is used as the AM antenna, which is isolated to ground through an inductor or coil. A separate FM antenna is incorporated on or in the glass above the heating grid, and the FM antenna is capacitively coupled to the heating grid.
A disadvantage of both approaches is the reduced sensitivity of the unheated antenna during snow and ice build-up. The heating grid melts snow and ice only in the area of the grid, and consequently snow and ice can accumulate on top of the unheated antenna. Such build-up reduces the sensitivity and performance of the unheated antenna by as much as 3 to 7 dB (decibels).
Further, use of the Lindenmeier design on increasingly larger rear windows results in an AM antenna with extremely high gain and sensitivity. This can be undesirable because additional countermeasures must be taken to reduce vehicle noise, overload, FM intrusion, power line noise, and radio settings (e.g. stereo/mono threshold, noise blanker, and frequency high cut). Such counter measures are costly and may also raise the noise floor of the system.